1. Field of the Invention
The present invention relates to computer programming languages having structured data types. More particularly, the invention concerns language processing techniques for supporting the initialization of variables based on structured data types with designated initializers.
2. Description of the Prior Art
By way of background, many programming languages have structured data types whose elements are defined by an ordered group of named fields, each usually having an associated type. The “struct,” “array” and “union” variables of the C programming language are examples. When declaring a variable that has a structured data type, many languages allow an initializer within the declaration that assigns initial values for the fields. Depending on the language, the initializer may contain only a group of values in the same order as the fields of the structured data type, called positional initializers, or it may contain a group of pairs of field names and corresponding values, called designated initializers.
Using positional initializers and relying on the order of the fields of the structured data type to initialize variables can in some cases prove error-prone and burdensome. An example would be a data structure whose elements are function pointers to a set of related operations, such as a set of file system operations for a file system. If the structured data type definition used to define the operations is modified so that the order of the fields changes, or if fields are removed or added (e.g., to remove or add operations), the meaning of one or more positional initializers in variable declarations based on this data type will likely change. Unless this leads to a type mismatch between a field and the corresponding value, language processors (such as compilers, interpreters, static analysis tools, semantic patch tools, etc.) cannot give an error, warning, or other diagnostic information when processing the variable declarations. Furthermore, whether or not language processors can issue a diagnostic, changing the fields of the structured data type will still require changing every positional initializer for declared variables of that type, which may prove prohibitive for large, complex software systems. Finally, some structured data types have many fields but users of those structured data types only utilize a smaller subset of the fields. For example, the file system data structure “struct file_operations” of version 2.6.24 of the Linux® kernel defines fields for 26 function pointers. However, most file systems do not implement 26 different file system operations. For example, the data structure “ext3_file_operations” for the Ext3 file system only fills in 12-13 of these function pointer fields (depending on the kernel configuration). Positional initializers for such variables must therefore include many placeholder values (such as NULL in the C programming language), and initializing the correct field requires counting out the correct number of placeholder values.
Designated initializers address these issues. By identifying fields by name, designated initializers will continue to work correctly even if the fields of the structured data type change, as long as the fields named in the designated initializer do not change. Furthermore, changing an existing field will only affect designated initializers that use the field, not necessarily all designated initializers. Finally, designated initializers remove the need for placeholder values.
Many large, complex systems make use of designated initializers for these reasons. For example, the Linux® kernel has long used C-language designated initializers, first via a GCC (GNU Compiler Collections) extension to the C89 standard, and later via the standard C99 syntax. For the reasons outlined above, many structured variables in the Linux® kernel should, by convention, only be declared with designated initializers, never positional initializers. Such data structures include, but are not limited to, the various data structures comprised of groups of function pointers representing related operations that may expand to include more operations at a later time. However, only convention and code review processes enforce the use of designated initializers.